Swimming pool acid cloud causes scare in Mill Valley

A Mill Valley neighborhood was shut down for more than an hour Friday morning after a swimming pool project sent a cloud of vapor into the air, authorities said.

The vapor was a by-product of muriatic acid, a fluid used to clean swimming pools, said Mill Valley fire Battalion Chief Scott Barnes, who discovered the cloud above a Lomita Drive home at about 8:40 a.m. The large, gray vapor cloud initially looked like smoke from a structure fire, Barnes said.

"The contractor claimed he'd never experienced something like this before," he said. "I immediately told them to shut their operation down."

Firefighters and police officers closed the street and advised residents of some 75 to 85 homes to stay inside because of the potential harm caused by inhaling the vapor, Barnes said. The contractor was also ordered to divert liquid containing muriatic acid from a storm drain.

The street was reopened shortly before 10 a.m. after the cloud dispersed, the Marin County Sheriff's Office said in a statement.

Firefighters notified the Sewerage Agency of Southern Marin and the California Emergency Management Agency of the incident, and the contractor was allowed to dispose of the liquid after neutralizing the acid with soda ash.

Todd Hendrickson, whose custom pool company Aquascape oversaw the project, said residents were not actually at risk because the acid was highly diluted.

"It's definitely not hazardous in the condition that it was in," he said. "Sniffing it right out of bottle, it would feel like it was burning your lungs."

Muriatic acid is commonly used to treat the surface of new pools or to adjust pH balance, he said. Typically the vapors disperse immediately but the lack of wind on Friday cause them to linger for much longer than usual.

"It happens in people's backyards every day all around the United States," he said of the use of muriatic acid.